The details can get a little complicated, but the idea is relatively simple.

If the first spouse who passes away does not have a large enough estate to have had to pay the estate tax, then there is an unused portion of the exemption limit leftover. The surviving spouse can take advantage of that. However, it is also available for anyone the surviving spouse might marry later.

Some people are apparently even advertising that they have this deceased spousal unused exclusion available on dating sites.

An estate planning attorney can advise you, if you did not take a spousal portability election when your spouse passed away and would like to now.